The present invention relates to hydrotreating of diesel fuels and in particular to improvement of those processes in a staged process.
The need to produce extremely clean transportation fuels is continually increasing. Future standards are being set, which cannot be achieved with existing process equipment. Although improved commericial catalysts are available, they are not sufficiently active to meet the increasingly more strict requirements for high quality commercial fuels, and thus modifications of process equipment are also necessary. Such changes in process equipment will be expensive and there is a need to identify novel processes to meet these requirements.
Overall Process Description
It is in the context of the above problems that the present invention was conceived. When the sulphur level must be lowered to less than 500 ppm sulphur, the conversions that are required involve desulphurization of highly substituted dibenzothiophenes, especially those in which the substituents are present on the aromatic rings adjacent to the heterocyclic sulphur atom. We will refer to such compounds as refractory sulphur compounds (RS-compounds). A typical example of such a compound is 4,6-dimethyldibenzothiophene (46 DMDBT). We have found that the conversion of the most refractory sulphur (RS) compounds (such as 46 DMDBT) in diesel fuels is made even more difficult by the presence of certain other components found in normal feeds to diesel hydrotreaters. Such compounds are referred to as inhibitors for hydrodesulphurization (HDS).
We have discovered that if such inhibitors are selectively removed from the feed and the feed containing less inhibitors is hydrotreated under typical commercial conditions used in today""s refineries, then the RS-compounds can be readily removed by hydrotreating using conventional catalyst loadings and process conditions. The degree to which the inhibitors are removed will depend on the particular adsorbent used and the cost of the removal process. In many instances, it is not necessary to remove all of the inhibitors to experience the benefits of our combined process. For ease of discussion, we will refer to diesel fuels which have been contacted with adsorbents for the inhibitors as xe2x80x9cinhibitor freexe2x80x9d diesel fuels, however, we do not mean to imply that 100% of the inhibitors have been removed. FIGS. 2 and 3 and Example 1 illustrate this point.
Hydrotreating Process
The hydrotreating step of the combined process scheme of this invention, shown in FIG. 1, can be any conventional hydrotreating process. This includes fixed or ebulated bed operations at conventional operating conditions such as temperatures in the range of 250xc2x0 C. to 450xc2x0 C., preferably 300xc2x0 C. to 380xc2x0 C. Pressures are also conventional such as 20-60 atm of hydrogen, and preferably below 40 atm of hydrogen. Higher temperatures and pressures will also provide the benefits of the present invention, however, lower pressures and temperatures are preferred to avoid yield losses of valuable diesel fuels and to avoid the need for construction of new process equipment in order to achieve extremely strict sulphur standards such as less than 300 ppm sulphur or even more strict sulphur standards of less than 50 ppm sulphur.
Catalysts used in the hydrotreating step are preferably those employed conventionally, such as mixed cobalt and/or nickel and molybdenum sulphides supported on alumina and mixed nickel and tungsten sulphides supported on alumina or silica. The combined process of this invention will also benefit newly developed catalysts such as those containing ruthenium sulfide and catalysts using novel supports such as silica-aluminas, carbons or other materials. For details on the state of the art in conventional hydrotreating processes, we refer to xe2x80x9cHydrotreating Catalysisxe2x80x94Science and Technologyxe2x80x9d, by H. Topsxc3x8e, B.S. Clausen and F. E. Massoth, Springer-Verlag Publishers, Heidelberg, 1996.
Inhibitor Removal Processes
It is possible to envision many ways of removing materials, which inhibit the hydrotreating process, especially the hydrodesulphurization of RS-compounds. However, the removal of inhibitors should be done in a practical way if this principle is to be realized commercially. The method used for inhibitor removal should be highly selective for only the inhibitors and should not remove the valuable components of the diesel fuel or other non-inhibiting components of the diesel fuel. An alternative process would be to selectively remove the RS-compounds as described in U.S. Pat. No. 5,454,933. However, in that patent the yield of diesel fuel was not specified, and in attempting to duplicate this patent, we have observed that the adsorbent carbon, though showing some selectivity for RS-compounds, has a high capacity for all diesel fuel components. When one attempts to recover the valuable diesel fuel components, the RS-compounds are also released, as the strength of adsorption is not high. Thus, it may be possible to concentrate the RS-compounds, but not remove them specifically. There are many different classes of materials that can inhibit the HDS of RS-compounds.
It is well known that certain basic compounds such as quinolines and acridines inhibit HDS reactions (see H. Topsxc3x8e, B. S. Clausen and F. E. Massoth, xe2x80x9cHydrotreating Catalysisxe2x80x94Science and Technologyxe2x80x9d, Springer-Verlag publishers, Berlin 1996; M. J. Girgis and B. C. Gates, Ind. Eng. Chem. Res., pp. 2021-2058, Vol. 30 No. 9, 1991; D. D. Whitehurst, T. Isoda and I. Mochida, Advances in Catalysis, pp. 345-471, Vol. 42, 1998; and references therein). However, any compound that will compete with RS-compounds for adsorption on the catalytic site will inhibit the HDS of the RS-compound. Thus, in addition to basic compounds, other strongly adsorbing species in the diesel fuel that is to be hydrotreated will lower the rate of removal of sulphur from the diesel fuel. We have found that such inhibitors are all highly polar materials that may be selectively removed from the hydrocarbons and RS-compounds by various adsorbents. By polar compounds we mean classical basic compounds such as were described above, including their benzo-analogs. These may be identified in diesel fuels by titration with strong acids in non-aqueous media. Other inhibitors include acidic nitrogen species, such as carbazoles, indoles and their benzo-analogs. Such acidic N-compounds can be identified by titration with strong bases in non-aqueous media. Still other inhibitors include amphoteric compounds such as hydroxyquinolines, and still other neutral compounds containing more than one nitrogen in an aromatic ring system or compounds which contain both oxygen and nitrogen in the same molecule. Further, inhibitors need not contain nitrogen, but may e.g. be composed of highly polar oxygen containing species.
Thus, it is possible to devise adsorption processes, which will selectively remove certain chemical classes of inhibitors or selectively remove essentially all inhibitor molecules by virtue of their polar nature. We have devised several different means to achieve selective inhibitor removal from diesel fuels using either their chemical properties or their polar properties. The particular method that is preferred will depend on the particular situation and the specific diesel fuel that is to be processed. However, the most preferred general method for inhibitor removal is based on their polar nature. The following text describes the various methods we have devised for use in the combined process of this invention.
Liquid Adsorbent Processes
In the present invention, our approach is to selectively remove the inhibitors for RS-compound conversion and then selectively desulphurize the inhibitor free feed in conventional HDS operations. We have found that only certain adsorbents have the selectivity desired.
Liquid adsorbents can be identified using their solvent parameters, fd, fp and fh, as defined by Teas [see J. P. Teas, xe2x80x9cGraphic Analysis of Resin Solubilitiesxe2x80x9d, J. Paint Technology 19, 40 (1968)]. To define the useful range of solubility parameters, it is customary to construct a triangular diagram and identify an area within the diagram in which the desired results are obtained. This effective area reflects the solubility characteristics of desirable solvents in terms of their solvent parameters, fd, fp and fh, which reflect the solvents"" dispersive, hydrogen bonding and polarity characteristics, respectively.
FIG. 4 shows the region of desired properties in the present invention. Solvents that have solubility parameters that fall within the desired range shown in FIG. 4 will be able to selectively remove the inhibitors, while rejecting the valuable diesel fuel components. In this example, dimethylformamide, dimethylsulphoxide and methanol containing 25% water are shown to fall within the desired area for our process.
In FIG. 5, examples of solvents that are not suitable for our process are shown. In this case water alone is a poor solvent for the inhibitors, while toluene and acetone are not selective for the inhibitors as they are good solvents for diesel fuel and do not form a separate phase.
A further example of how two non-useful solvents may be combined in specific proportions to make a mixture, which has the correct solvent properties, is shown in FIG. 6. In this example, n-propanol is a borderline adsorbent as it is too strong a solvent for the desired inhibitor free fuel and water is too poor a solvent for the inhibitors. A mixture of the two falls within our desired range of solvent parameters.
A further advantage of some mixtures is that some specific combinations form azeotropes (constant boiling mixtures), which have the desired solvent parameters. This is the case shown in FIG. 6, where the azeotrope of water and n-propanol consists of 71.8% n-propanol and 28.2% water. This azeotrope boils at a lower temperature than either component and would thus retain constant composition in the distillation step used for solvent recovery.
Another important property of the solvent used to remove inhibitors is the density of the liquid. To operate a successful separation process, the density of the solvent should have a lower or higher specific gravity than the diesel fuel being treated. The difference between the specific gravity of the diesel fuel and the solvent should be at least 0.02 specific gravity units, and preferably more than this value.
To select a pure solvent or mixture, which has the correct solubility parameters for use in our process, one only needs to find the solvent parameters of the solvent or components of the mixture (from the literature or by experimental determination) and plot them on a triangular diagram similar to the one shown in FIG. 4. If the plot of the solubility parameters falls within the desired area of FIG. 4 then material will be useful for the process of this invention. If a mixed solvent is to be used, the mixed components should constitute a single phase in order to effectively extract the inhibitors from the diesel fuel.
To recover the solvent for reuse, distillation or, in some instances, a simple flash process can separate the solvent from the dissolved inhibitors. The isolated inhibitors may be disposed of by burning or in some cases they may serve as sources of chemicals.
The process used for removing inhibitors with a liquid adsorbent can be any conventional process used for liquid-liquid extraction such as columnar counter current flow, stirred tank, hydroclone, etc. It may also be staged to increase the efficiency or it may be a single contact process, depending on the degree of separation desired. An illustration of this stage of our invention for selectively removing inhibitors with liquid adsorbents is given in FIG. 7. The schematic diagram illustrated in FIG. 7 represents a columnar countercurrent flow process. This illustration is only one example of a process that can be used to selectively remove the inhibitors prior to hydrotreating, but should-suffice to instruct anyone skilled in the art as to how to conduct such a process.
The range of conditions, which may be used in this extraction process, is quite broad and will depend on the particular solvents used and hydrotreating feeds that are being treated. Ambient conditions are preferred, but in some cases the efficiency of inhibitor removal or the density difference between the solvent and the diesel fuel may be optimized by raising or lowering the temperature. However, the temperature should not be higher than the boiling point of either the diesel fuel or the extraction solvent, and the temperature should not be lower than the freezing or pour point of the diesel fuel or extraction solvent. For ease of extraction solvent recovery, the boiling point of the extraction solvent should be considerably different from the diesel fuel boiling range, and preferably the solvent should have a boiling point lower than the lowest boiling component of the diesel fuel, or the lowest boiling inhibitor in the diesel fuel.
Solid Adsorbent Processes
Another version of the present invention is to use a solid adsorbent in the inhibitor adsorption step. In this mode, many process variations are possible. The adsorption process may be conducted in a fixed bed operation or in moving beds, such as fluidized beds, ebulated beds, or simple moving beds. FIGS. 8 and 9 illustrate two examples of such processes. For all cases in which solid adsorbents are used, three integrated steps are needed for the overall process. Firstly, the solid adsorbent is contacted with the diesel fuel to remove inhibitors. Secondly, the solid is separated from the physically adsorbed inhibitor free fuel. Thirdly, the solid adsorbent, containing strongly held inhibitors is regenerated to provide inhibitor free adsorbent, which is reused.
In FIG. 8 two fixed beds are shown, in which one is in the adsorption mode, while the other is in the regeneration mode. The inhibitor free fuel is predominantly separated from the solid adsorbent by merely passing the diesel fuel through the fixed bed of adsorbent. However, a small amount of inhibitor free fuel is retained on the adsorbent at the end of the adsorption cycle, and this inhibitor free fuel is recovered prior to the regeneration step. Inhibitor free recovery is achieved by a stripping operation with a hot gas such as steam, hydrogen, refinery gaseous fuel, or other refinery gases produced as byproducts from another refinery process. The stripping operation can also be conducted with a light liquid, such as a C4-C7 hydrocarbon, but this stripping liquid should then be recovered with some stripping gas, prior to the adsorbent regeneration.
In this mode of inhibitor adsorption, the diesel fuel constitutes a liquid phase. The preferred temperature range for this mode of operation is from ambient to slightly below the initial boiling point of the diesel fuel being treated. This temperature range is generally between 15xc2x0 C. to 300xc2x0 C., but could also be conducted at sub-ambient temperatures if desired. The preferred range is from 20xc2x0 C. to 200xc2x0 C.
The adsorption cycle length is determined by the capacity of the adsorbent to remove the inhibitors from the diesel fuel feed. This is generally determined by analysis of the inhibitor free fuel for N-compound content. The preferred level of nitrogen in the treated inhibitor free fuel is generally below 200 ppm, and, more preferred, the level should be below 100 ppm, or even more preferred less than 20 ppm. At this level of nitrogen in the inhibitor free fuel, the subsequent hydrodesulphurization (HDS) is quite facile, and levels of sulphur in the product of less than 20 ppm can be achieved under mild conventional process conditions including lower pressures such as 30 atm of hydrogen as will be shown in the examples.
In another embodiment of this invention, the adsorption step can be conducted at elevated temperatures, where the diesel fuel is in the vapor phase. This temperature should be high enough for the diesel fuel to be in the vapor phase, but low enough for the cracking of the valuable diesel fuel not to occur. The temperature should also be low enough such that inhibitors are adsorbed by the solid adsorbent and are not released back into the inhibitor free fuel stream. In this mode of operation, the temperature range is generally from 300xc2x0 C. to 450xc2x0 C., and the preferred temperature range is from 350xc2x0 C. to 400xc2x0 C. When operating in this mode, the inhibitor free fuel is not substantially adsorbed by the solid adsorbent, and the stripping operation may in some instances not be necessary. The regeneration of the adsorbent is conducted as described above, when the adsorbent""s capacity for removing the inhibitors has been reached. In this mode, the level of nitrogen in the effluent, inhibitor free fuel, again determines the capacity of the adsorbent to remove inhibitors. As described above, the level of nitrogen in the effluent is preferably below 200 ppm and even more preferred below 20 ppm.
In the regeneration step, it is preferable to restore the solid adsorbent""s capacity, so that it may be recycled back to the adsorption zone and reused. Such regeneration can be either oxidative, i.e. by burning in a fixed bed operation, or reductive. In some cases it may be desirable to heat exchange the regenerated hot solid adsorbent, either directly or indirectly to recover the heat from the combusted polar compounds and/or to cool the adsorbent to the desired temperature for the adsorption zone. For hydrogenative regeneration, the inhibitors adsorbed on the adsorbent may be removed by high temperature contact with a gas containing molecular hydrogen, such as pure hydrogen or a refinery off gas containing a substantial portion of molecular hydrogen. This contact with hydrogen can be done at atmospheric or elevated pressures, but it is essential that the temperature be above 400xc2x0 C. These regeneration procedures should be conducted at temperatures, which do not lower the surface area of the solid adsorbent, but substantially remove all of the inhibitors as gaseous products. The preferred temperature range for these regeneration steps is from 400xc2x0 C. to 1000xc2x0 C., and even more preferred, between 500xc2x0 C. to 700xc2x0 C. In some instances, the solid adsorbent may contain catalytic additives, which enhance the regeneration process. For example, in oxidative regenerations, oxidation catalysts such as calcium, magnesium, iron, potassium or sodium may be added, and in such instances, the preferred combustion temperature is 350xc2x0 C. to 500xc2x0 C. Hydrogenative regenerations may be enhanced by hydrogenation catalysis, such as nickel, iron, platinum, palladium, or other group VIII metals.
Other embodiments of this invention include inhibitor removal steps, in which the solid adsorbent is continuously circulated between the adsorption step in one vessel and the regeneration step in a separate vessel. Such processes include moving beds, ebulated beds, hydroclones, fluidized beds, etc. with external regeneration. These moving bed processes can be stand-alone operations or can be integrated with existing refinery equipment. In one preferred embodiment of this invention, the inhibitor removal step is integrated with an existing FCC operation in the refinery. In this embodiment, the adsorbent comprises the steady state, or equilibrium, FCC catalyst. FIG. 9 illustrates one example of this type of integrated process. As can be seen in the figure, the equilibrium catalyst is taken out as a side stream just after regeneration and is then contacted with the diesel fuel feed that contains inhibitors. The inhibitor free fuel is separated from the FCC catalyst adsorbent and then hydrotreated to remove sulphur contaminants as described above. The FCC adsorbent, containing inhibitors and some physically adsorbed inhibitor free fuel, is returned to the FCC operation in the stripper zone, where the inhibitor free fuel is recovered as part of the FCC product stream, and the inhibitors are retained by the FCC adsorbent. The FCC adsorbent is admixed with FCC catalyst containing coke produced in the FCC process and both are regenerated by combustion in the FCC regenerator. In such an integrated process, the relative amounts of equilibrium catalyst that are taken for the inhibitor adsorption process and returned to the FCC cracking process are determined by the content of inhibitors in the diesel fuel feed and the capacity of the equilibrium catalyst to remove-those inhibitors.
As the regenerated FCC catalyst often exits the regenerator at temperatures in excess of 900xc2x0 C., it is sometimes desirable to cool the adsorbent FCC catalyst stream before the adsorption step in order to avoid cracking of the diesel fuel. This can be accomplished by either direct heat exchange with steam or refinery gas or by indirect heat exchange with water which produces steam for refinery heat or power generation. The degree of temperature reduction will depend on which mode of operation is employed in the adsorption step as described above.
Another embodiment of this invention is shown in FIG. 10, where the adsorbent and fuel to be treated are contacted in a conical circulating vessel, such as a hydroclone. Such types of vessels are highly effective in separating solids and liquids at high throughputs. In such a process, the critical features include contact times between the solid and liquid sufficient to achieve the desired level of inhibitor reduction, and flow velocities for liquid and solid which can achieve separation of liquid and solid without carryover of solid into the liquid exit stream. It is within the scope of this invention to conduct such adsorption processes at room temperature, at elevated temperatures or sub-ambient temperatures, depending on the nature of the adsorbent, the nature of the fuel that is being treated and the desired result of the treatment. Anyone expert in this area can easily determine the optimal conditions by experimental studies.
Suitable Solid Adsorbents
As will be shown in the examples, the choice of a suitable solid adsorbent for inhibitors is the key to the success of this combined process. We have found that many porous solids, when contacted with diesel fuel feeds, can provide some benefit to the HDS of refractory sulphur compounds (RS-compounds). However, the solid adsorbents of choice should not only have the capability of removing inhibitors, but they should be highly selective in this removal and should have capacities for removing substantial amounts of inhibitors before they are no longer effective. Once the inhibitors have been adsorbed, the adsorbent should have the properties that allow the recovery of physically adsorbed inhibitor free fuel, while strongly retaining the adsorbed inhibitors. In addition, the preferred solid adsorbents should have the durability to withstand regeneration in a process in which the adsorbed inhibitors are burned off of the adsorbent without losing their effectiveness in multiple cycles of adsorption/regeneration.
It is possible to remove inhibitors using organic solid adsorbents, and the use of such materials falls within the scope of this invention. However, regeneration of such materials is more complicated than for inorganic solids as combustion is not a viable option. Suitable solids include porous carbons and intrinsically porous ion-exchange resins (so-called macroreticular resins). As will be shown in the examples, both strongly acidic and strongly basic ion-exchange resins adsorb some inhibitors from diesel fuel feeds, and such treatments of diesel fuel feeds allows a higher degree of HDS of RS-compounds than is possible for untreated diesel fuel feeds. However, regeneration of the ion-exchange resins, to restore their original capacity for inhibitor removal, requires large volumes of reagent liquids (e.g. aqueous or alcoholic acids and bases) to remove the chemically adsorbed diesel fuel components and restore the active sites within the ion-exchange resin. Carbon adsorbents have similar disadvantages in that they are not highly selective for only the inhibitors and they cannot be regenerated by burning.
Another class of materials, which has been found to be effective for the combined process of this invention, is porous strongly basic alkaline earth oxide containing materials. Examples of such materials include carefully calcined magnesium hydroxy carbonates and porous Portland cements. These materials have the additional advantage that they can function as oxidation catalysts, which allows the use of lower temperatures in the regeneration step.
The most effective solids, which have been identified for this application, are acidic silica/alumina containing materials having surface areas greater than 100 m2/g. Such materials include pure silica/aluminas produced by co-precipitating silica and alumina from a variety of precursors as well as composites containing said silica/aluminas in combination with other materials, such as zeolites. The acidity of these adsorbents can be conveniently measured by the well-known xe2x80x9calphaxe2x80x9d test as described by Weisz and Miale, J. Catal. 4, 527 (1965). This test measures a solid""s ability to crack hexane at atmospheric pressure and 538xc2x0 C. Normal silica/aluminas containing about 4% aluminum have alpha values of 1, whereas composites containing zeolites can have alpha values exceeding 100. For the purposes of the present invention, it is preferable to utilize solid adsorbents having alpha values of from 0.5 to 10 and most preferred from 1-5. Such materials are often used as catalyst supports or as composite catalysts. They are highly durable and may be regenerated many times without losing effectiveness in the application of the present invention. This is especially true for FCC cracking catalysts in which the binder or matrix (silica/alumina) comprises about 60% of the composite and an acidic zeolite comprises the rest of the composite. The acidity of such composites can be improved by impregnation of or co-precipitation of the silica/alumina with phosphorus containing acids prior to the final calcination step as described in U.S. Pat. Nos 3,962,364, 4,044,065, 4,454,241 and 5,481,057. Such phosphorus treated silica/alumina containing composites are also preferred materials in the present invention.